1. Field of the Invention
The present invention relates to a negative working imaging process employing photosensitive microcapsules. More particularly, the present invention utilizes a reversal phenomenon and two exposure steps to actinic radiation to enable the production of negative images from photosensitive microcapsules.
2. Description of the Prior Art
Photosensitive imaging systems employing microencapsulated radiation sensitive compositions are the subject of commonly assigned U.S. Pat. Nos. 4,399,209 and 4,416,966 to The Mead Corporation as well as copending U.S. patent application Ser. No. 320,643 filed Jan. 18, 1982. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photosensitive composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color precursor. Image-wise exposure hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is subjected to a uniform rupturing force by passing the sheet through the nip located between a pair of pressure rollers.
U.S. Pat. No. 4,399,209 discloses a transfer system in which the imaging sheet is assembled with a developer sheet prior to being subjected to the rupturing force. Upon passing through the pressure rollers in contact with the developer sheet, the microcapsules rupture and image-wise release the internal phase whereupon the colorformer migrates to the developer sheet where it reacts with a dry developer and forms a color image. The imaging system can be designed to reproduce monochromatic or polychromatic full color images.
U.S. Pat. No. 4,440,846 discloses a so called "self-contained" imaging system wherein both the image-forming agent and the developer material are located on the same substrate. In the system according to U.S. Pat. No. 4,440,846, the image-forming agent is encapsulated in a layer of pressure rupturable capsules, and the subsequent exposure and capsule rupture causes the image-forming agent to contact and react with the developer to produce an image on the substrate.
The imaging materials and processes described in the aforementioned references are by nature positive-working (i.e., the image produced is an exact copy of the original image). It would be desirable to design an imaging system employing photosensitive microcapsules which are negative working. In such a system, the image produced would be a complete inversion of the original image. For example, it would be desirable to produce high quality negative image copies of originals such as microfilm and microfiche which, are by nature, "negative-type" originals.
Several methods have been proposed to produce negative images.
One method described in the aforementioned publications is to incorporate a photosoftenable or photodepolymerizable material in the microcapsules. Although such a system does produce negative images, the system has been difficult to commercially implement.
In silver-halide photographic systems, reverse image processes are well known. They may take the form of several methods, including the bleach/re-expose technique and transfer diffusion procedures. Such a system does not utilize photopolymerizable microcapsules.
Another way to produce negative images is to utilize a photopolymerizable composition containing a nitroaromatic compound which may be converted to a photoinhibitor. The formed photoinhibitor acts to prevent photopolymerization of an ethylenically unsaturated compound capable of addition polymerization by free-radical initiated chain propogation. The inhibitor is produced by exposure of the composition to actinic radiation of a designated wavelength for a designated time period. Such a system is disclosed in U.S. Pat. No. 4,269,933.
In the context of a photohardenable or photopolymerizable system, a negative image may be produced wherein the areas of a photosensitive layer corresponding to the dark or opaque areas of the original are photohardened, while the areas corresponding to the light or transparent areas of the original are not substantially photohardened. The negative images are obtained by utilizing a reversal process. Such a process has not been disclosed with use in connection with photosensitive microcapsules.
For a photosensitive composition, sensitomeric behavior is represented graphically by an H&D curve. FIG. 1 is a typical H&D curve depicting the behavior of a positive working system. As seen in FIG. 1, the density of the image decreases with increasing exposures. By comparison, an H&D curve for a photosensitive material which exhibits reversal on a similar exposure scale is depicted in FIG. 2. In this system, the H&D curve contains two distinct portions: a first, "positive" area where the system obeys the behavior as shown in FIG. 1, and a second "negative" area where the density of the image increases with increasing exposure.
In such a system as shown in FIG. 2, positive or negative images can be produced depending on how the exposure intensities and exposure times are manipulated. When desiring to produce negative images, the process steps are controlled so that only the "negative" portion of the H&D curve is utilized.
For example, U.S. Pat. No. 3,380,825 discloses a reverse-image process employing photohardenable compositions in the photosensitive layer. This process includes a first image-wise exposure under such conditions that a gaseous polymerization inhibitor exhausts the photoinitiator that had been excited by actinic radiation in the exposed areas before any substantial polymerization had occurred. The photosensitive layer is then shielded from the gaseous inhibitor during a second, nonimage-wise exposure to the same source of actinic radiation. Polymerization occurred only in those areas where the photoinitiator was not exhausted by the first image-wise exposure. Implementation of this process on a commercial scale has been unsuccessful due to the careful and inconvenient manipulations that are required to produce reverse images of excellent quality.
U.S. Pat. Nos. 3,782,951 and 3,888,672 disclose a photopolymerizable process capable of yielding a reverse image. The process includes utilizing a photopolymerizable composition containing an ethylenically unsaturated monomer, an organic polymeric binder, and, as an initiator system, a lophine dimer and a hydrogen-donor compound. The composition is subjected to a first, intense image-wise exposure and then a second, nonimage-wise, less intense exposure. By utilizing this process, negative images are produced. The images are produced as a result of utilizing the so-called reversal phenomenon whereby the first exposure step deactivates the photopolymerizable composition in the image-wise exposed areas. Success of this process is dependent upon five variables. These variables are: concentrations of hydrogen-donor compounds, lophine dimer, ethylenically unsaturated monomer, and organic polymeric binder, and the intensity of exposure. Accordingly, production of good quality negative images requires manipulation of the above-mentioned five variables.
Accordingly, there exists a need in the art for producing negative images by utilizing the reversal phenomenon in conjunction with photopolymerizable microcapsules.
In addition, when producing copied images by utilizing a positive-working system employing photosensitive microcapsules, it is necessary to selectively expose the background areas of the imaging sheet to prevent unwanted image formation on the developer sheet. A negative-working system would be more desirable than a positive-working system since the image-wise exposure step would only be required in the areas in which images are to be produced. Such a system is referred to in the art as a "direct-write" system and is particularly useful for producing photocopies from originals containing only written text. A direct-write system has not been developed which utilizes photosensitive microcapsules and the reversal phenomenon.
Accordingly, there exists a need in the art for a direct-write copying system utilizing photosensitive microcapsules.